Before beginning an analysis of net forces in order to study the motion or the equilibrium of a system, one must first define…the system. Only forces exterior to the system are to be considered thereafter.
Note also that for systems at rest, or for those in uniform linear motion, the fundamental principle of dynamics (Newton’s First Law) states that the sum of the exterior forces acting on the system must be zero.
Select the system you want to work with.
The external forces will automatically appear.